Probably a pretty boring answer, but speeding. If they built an autobahn type highway up here in the great white north, I’d definitely take advantage.
Other than that, there isn’t really anything (except maybe Bank Robbery, but the fact that’ it IS illegal is most of it’s appeal to me). I don’t like the way drugs make me feel or even copious amounts of alcohol for that matter.
Public nudity really isn’t my thing.
Income tax evasion. 
Not me, I went to the hospital three times last year for rather expensive tests. I like that the government foots the bill. I’d feel guilty not paying.
It’s a good thought though…
Kill a couple people that need a good killing.
Just for the experience, pay an obscene amount of money (which I’d steal) to have some ravishing hooker spend a week with me on a beach somewhere.
Invade people’s privacy.
Well, maybe I wouldn’t do that so much now, but in the pre-internet days I would’ve done it a lot.
I’d probably try pot. Just once, to see what it was like.
I’d drive around and shoot drug dealers.
I might try one of the drugs that’s supposed to give you a real mind-altering experience, like LSD. I’ve always been curious about what it would be like, but fear of both legal reprisal and long-lasting effects to my mind (not to mention not having the faintest clue where to acquire such things nor interest in doing so) have kept me from even considering it.
If it were legal and I could do it in a controlled environment, I might be tempted. Once.
I’d go into London and do a huge shit on the sidewalk, then as people walked past, I’d point to it and hold out a cup, saying “penny for the shite?” Then I’d hit them in the face.
Or perhaps not.
I would occasionally drive really fast, over 120 mph like I use to do when I was in my early 20s.
I would occasionally do LSD or Shrooms again, like I use to do when I was in my early 20s.
I would probably still smoke pot occasionally, like I use to do before I went in the Navy and for a little bit of time after I got out.
I can’t think of any other illegal activity I really would want to do. Not even Income Tax evasion, I still believe in this country and I am willing to put in my share.
Shooting at **Mangetout’**s crotch with a BB gun.
Put your clothes back on!
I am extremely uninterested in drugs, so I’ll say: plural marriage. I’ve known a few Neo-Pagan people who were quite happy in relationships that were plural marriages in all but name; if they’d been able to make it legal, they would have been overjoyed.
(Re: topless. Going topless is legal for women in Ontario, and… they don’t. In fact, I saw someone in a burqa at College Park this week. )
“i’ve got a little list, they never will be missed.”
Throw water balloons or something equally attention getting without causing damage at people who are jerks on the road. Every car who cuts someone off, tailgates, doesn’t use their turn signal, or is otherwise an asshat would be pelted by me.
Yeah, legality doesn’t slow me down much–however, I’d like to be able to landscape my yard any way I want and that would include kind pot plants AND opium poppies (they’re pretty and quite useful.)
I wouldn’t wear clothes much around my house and yard.
I’d drive like a mad bastard at every opportunity. I LIKE going fast and I know how.
I’d keep goats and/or sheep and a pony in my yard. I’d raise rabbits for food, too.
I’d make any and all changes and improvements to my house I want–fuck the permit laws, hell yeah!
I’d let my dog run around off leash most of the time–he’s small, well behaved and stays right close to me anyway so a leash is just silly for him.
I’d go ahead and run with a dynamic connection on the router.
I probably wouldn’t kill anyone (probably!) but I’d definitely find myself slapping people briskly across the face on a fairly regular basis.
I’d drive to Crawford, TX and take a giant shit on somebody’s doorstep.
Heck, I’d drive my van all over Arizona, just 'cuz they won’t let me do it now.
Hmmm, I better stop before my brain gets all busy thinking about stuff like this…
Like others have said, pot. Maybe try mushrooms or LSD just to see what it’s like.
Were I living in the US, I’d probably be an occasional pot-smoker regardless of the legality, but here not only is the base penalty higher, there’s also deportation on top of it. So I’ll pass for now.
Why can’t you drive your van in Arizona?
Oh, thought of another one: I would have a pet ocelot. It used to be legal, back in the 60s–I have a couple of books from that era about the care and feeding of pet ocelots. Really, the only thing that’s stopping me from having one now is that I live in the city and it’s not legal. I’ve loved them since I was a kid.
I’d say “drive really fast,” but I do that a lot anyway.
If I did that, the next post in this thread would be from Mrs. Bricker, and you can guess which formerly-illegal act SHE’D be doing.
There’s more than a few people who’d be due for a punching.